1. Technical Field
The present disclosure relates to films/coatings (e.g., catalyst films/coatings) and assemblies/methods for fabricating films/coatings and, more particularly, to assemblies and methods for fabricating or synthesizing catalytic material (e.g., catalytic nanostructures) in flame and depositing the catalytic material onto substrates.
2. Background Art
In general, efficient energy generation and storage is one of the most important issues to solve for the 21st century. Within this, electrochemical devices are expected to play a significant role as their performance is typically not limited by traditional thermochemical cycles (e.g., Carnot and Rankine cycles), and this is expected to lead to electrochemical systems with ultra high efficiencies. For example, the maximum achievable efficiency of a general proton exchange membrane (PEM) fuel cell operating at 80° C., ε=ΔG/ΔH, is about 93%. This compares favorably to only 63% for the Rankine cycle (though only about 40% is typically achieved in practice), which is responsible for approximately 90% of the global electricity production. Of course, fuel cells require high purity hydrogen fuel. Hydrogen is also one of the most highly utilized industrial gases and can also be employed as an energy carrier or means for storage of intermittent renewable energy (e.g., PV solar, wind, etc.). This makes the high efficiency generation of hydrogen a top priority, particularly when it is considered that water electrolysis to hydrogen and its use as a fuel is a potentially sustainable energy cycle that would lower a country's dependence on foreign oil. Insufficient kinetics for the oxygen reduction reaction (ORR) at the PEM fuel cell cathode in acid media on state-of-the-art electrocatalysts and the current requirement for relatively high loading of Pt for the hydrogen evolution reaction (HER) at the PEM electrolyzer cathode leads to low Pt mass activity of both unsupported and carbon supported Pt (Pt/C) catalysts. This low mass activity remains one of the most serious challenges for the mass deployment of proton exchange based electrolyzers and fuel cells.
Three of the most significant challenges facing the wide commercialization of the proton exchange membrane fuel cell (PEMFC) are: i) improving catalyst tolerance to impurities; ii) simplifying water and thermal management schemes; and iii) enhancing the kinetics of the oxygen reduction reaction (ORR) at the cathode. Research over the past decade has focused on mitigating the above challenges by increasing the operating temperature of PEMFCs from about 80° C. to greater than 120° C.; however, operating at elevated temperatures introduces new difficulties in maintaining the proton exchange membrane in a state of adequate hydration. In general, some phosphoric acid-doped polybenzimidazole (PBI) membranes have been the most successful. PBI outperforms conventional membranes (e.g., Nafion®) by self-solvating protons to allow charge migration, hence minimizing the reliance on water for proton transport.
In general, the heart of the PEMFC is the membrane electrode assembly (MEA), which typically consists of the membrane sandwiched by the active catalyst layers. State-of-the-art MEA manufacturing is a multi-step, energy and manpower intensive process. MEAs also typically contain a high loading of noble metal catalysts which must be mined and processed to high levels of purity. Some entities uses a single-step MEA manufacturing approach for high temperature PEMFCs (HT-PEMFC). At the catalyst level, PEMFCs generally have three significant limitations: 1) low Pt utilization, leading to high loadings and high cost; 2) Pt agglomeration, leading to device performance degradation; and 3) support corrosion, limiting high temperature operation. One factor with regards to low Pt utilization in current PEMFCs is the lack of guiding principles for the rational design of electrodes, which involves organizing the catalyst, support and ionomer to balance complex relationships between electrochemically active area, reactant mass transport, electron transport and current collection.
A variety of approaches have been employed in attempting to address these issues. One approach involves increasing the overall surface area available for reaction by forming metal particles with nanometer-scale dimensions. However, a primary challenge with the use of nanoparticulate electrocatalysts is that these zero-dimensional (OD) morphologies possess proportionally higher numbers of defect sites, lattice boundaries, and low coordination atoms at their surfaces. Inherently, defect sites are substantially less active towards oxygen reduction reaction than defect-free crystal planes, largely because of differences in the local coordination geometry and surface energy, which can directly influence the interfacial interaction between the metal surface sites and the adsorbed oxygen species.
Core-shell nanoparticles present a unique mechanism by which catalyst researchers can tune activity for a wide array of chemical and electrochemical processes. Because the core composition shifts the electronic structure at the surface of the active shell material, core-shell structured nanoparticle catalysts can be tailored to optimize the surface activity and/or product selectivity for selected applications. However, to take advantage of modified electronic structures for tuning catalytic activity, tight control of the shell growth to one or two monolayers may be required. For Pt and other precious metal shells, some experimental and theoretical studies over the past decade have been devoted to investigating the role of underlying supports on the catalytic and electrocatalytic activity of Pt and other noble metals. A picture of the theory behind metal-on metal core-shell particles is starting to emerge; however, the coupled geometric and electronic effects of non-metallic cores (such as carbides or oxides) on precious metal shell activity is not well understood. While experimental studies with 1-2 monolayer Pt shells have indicated the promise of such systems for enhanced electrochemical activity, advances in the fundamental understanding of metal/non-metal interactions can provide guidance on core-shell nanoparticle design.
Fabrication of core-shell catalysts has sometimes been based on the use of passivating ligands in colloidal suspensions. In general, passivation is important for controlling shell growth to maintain adequately thin shells for tuning surface reactivity associated with the core-shell structure, but the remaining ligands can inhibit catalyst functionality, particularly for systems like electrochemical reactions where catalyst/support interactions are important. Post-processing thermal treatments to remove the ligands can require excessive temperatures that can disrupt the preferred core-shell structure. To substantially eliminate ligands from core-shell fabrication, researchers have implemented electrochemical techniques such as voltammetric surface dealloying, underpotential electrodeposition with shell metal displacement in liquid solutions, or gas-phase synthesis with atomic-layer deposition on fluidized particles. However, expensive batch processes do not provide an economic approach for industrial scale catalyst manufacturing needed for many energy-related electrochemical and chemical conversion applications. Developing scalable fabrication processes that provide the necessary control of nanoparticle structure for enhanced activity presents significant techno-economic challenges for bringing core-shell nanoparticles to large-scale industrial catalytic applications.
Thus, an interest exists for improved films/coatings (e.g., catalyst films/coatings), and related assemblies/methods for fabricating improved films/coatings. These and other inefficiencies and opportunities for improvement are addressed and/or overcome by the assemblies, systems and methods of the present disclosure.